# Coding a sequence into a natural number by map $f$ with $f(k_1, .., k_n) + f(k_{n+1}, .., k_{2n}) = f(k_1 + k_{n+1}, .., k_n + k_{2n})$

Has anyone discovered a way of coding a sequence of natural numbers into a natural number by map $f$ that has the following property $f(k_1, .., k_n) + f(k_{n+1}, .., k_{2n}) = f(k_1 + k_{n+1}, .., k_n + k_{2n})$? ($k_1$ and so on are natural numbers and $f$ codes any sequence of natural numbers into a unique natural number.)

Add: Let us say that we limit $n$ to be a particular number. So if we set $n$ to be 3, there cannot be $f$ that accepts one number only and so on.

Add 2: Every number received by $f$ is treated as numbers, not sequences.

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But any such $f$ must satisfy $f(f(1,1))=f(1,1)f(1)=f(f(1),f(1))$, so $f$ cannot satisfy the additive property and code any sequence into a unique natural number simultaneously. – Ivan Loh Mar 19 '13 at 3:40
For $n=1$, take $f(n)=n$. For fixed $n>1$, @Alex Kruckman has answered it completely. – Ivan Loh Mar 19 '13 at 3:50
No, but then I added that let us assume that $f$ only takes $n$ numbers; it does not take less than $n$ number or more than $n$ numbers. – Maps from subways Mar 19 '13 at 4:29

Such a coding is impossible for $n>1$.

Let $a = (1,0,\dots,0)$ and let $b = (0,1,0,\dots,0)$.

When I write $m \cdot a$ for a natural number $m$ and a vector $a$, I mean $a + \dots + a$ (added $m$ times).

Now $a$ is coded by a natural number $m_a = f(a)$ and $b$ is coded by a natural number $m_b = f(b)$. But:

\begin{align} f(f(b),0,\dots,0) &= f(m_b\cdot a)\\ &= m_b\times f(a)\\ &= m_a\times f(b)\\ &= f(m_a\cdot b)\\ &= f(0,f(a),0,\dots,0) \end{align}

...which shows that the coding is not injective, since $(0,f(a),0,\dots,0)\neq (f(b),0,\dots,0)$, unless $f(a) = f(b) = 0$ but then again we have that the coding is not injective, since $a\neq b$.

Edited: Maybe an explicit example will help you see what's going on. Let's take $n = 2$ and say for example that $f(1,0) = 3$, $f(0,1) = 5$. Then:

\begin{align} f(5,0) &= f(1 + 1 + 1 + 1 + 1, 0 + 0 + 0 + 0 + 0)\\ & = f(1,0) + f(1,0) + f(1,0) + f(1,0) + f(1,0)\\ & = 3 + 3 + 3 + 3 + 3\\ & = 15\\ & = 5 + 5 + 5 \\ &= f(0,1) + f(0,1) + f(0,1) \\ &= f(0+0+0,1+1+1) \\ &= f(0,3) \end{align}

If you compare these steps with my argument above, you'll see that it's exactly the same, except $3$ is $m_a$ and $5$ is $m_b$.

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But what if we do not allow $f$ to receive only one number? Assume that we don't use the coded number to another function; every number received by $f$ is treated as numbers, not sequences. – Maps from subways Mar 19 '13 at 4:30
Can you explain your comment further? In what I've written, I've always treated $f$ as a function $\mathbb{N}^n\rightarrow\mathbb{N}$, that is, I've only applied $f$ to sequences of length $n$. – Alex Kruckman Mar 19 '13 at 4:32
But $1, 0, 0, \ldots , 0$ is a sequence of $n$ natural numbers. What is wrong with that? – Ivan Loh Mar 19 '13 at 4:33
What I mean is, $f(f(b), 0, ..., 0) = f(m_b \cdot a)$ part. So, $f(f(b), 0, ..., 0) = f(m_b, 0, ..., 0)$. And as $a$ represents $(1, 0, ..., 0)$, this is used to equal to $f(m_b \cdot a)$. But while $a$ does represent $(1, 0, ..., 0)$, I want $f$ to disallow one-number input case. So $f(m_b \cdot a)$ is not allowed, and any numbers (input) in $a$ are treated as numbers, not sequences. – Maps from subways Mar 19 '13 at 4:58
Oh, I saw the edited part now. I'll have a look at it and respond again. Thanks. – Maps from subways Mar 19 '13 at 5:01